For strength and forearm growth, you need to train all three types of grip strength: support, crushing, and pinching. Use these five exercise categories:

1. Heavy compound movements that emphasize support-grip strength.

This includes overhand-grip deadlifts, snatch-grip deadlifts, Reeves deadlifts, weighted pull-ups, heavy rows, single-arm dead hang holds, heavy shrugs, and farmers walks.

These should involve significant time under tension and would obviously be performed without straps.

2. Crushing and pinching movements.

This includes fat-grip exercises, plate pinching drills, towel pull-ups, ledge or mountain climber pull-ups, rope climbing, rope pulling, and hex dumbbell pinching exercises.

These build significant functional strength and hypertrophy throughout the entire musculature of the forearms, not to mention Herculean grip strength.

3. Bottoms-up exercises

Few exercises obliterate the forearms, hands, and finger muscles to the extent that these do. You'll be required to activate every muscle in the hands and forearms to stabilize an unstable kettlebell or plate. Here's one example:

Bottoms-up exercises also teach grip stability and motor control in the fingers, wrists, and hands.

4. Direct forearm isolation training.

This includes wrist curl variations, wrist roller drills, and leverage based forearm drills with a sledgehammer or mace. While these drills typically involve little else but the actual forearm and grip muscles, few exercises will directly target the wrist flexor and extensor muscles to the same degree.

In terms of training economy, these probably aren't ideal if you're only able to make it to the gym once or twice per week. But if maximal forearm development is the goal you'll want to include both extension and flexion based forearm isolation movements in your training.

5. Bicep work that targets the brachioradialis muscle of the forearms.

This includes exercises such as hammer curls, reverse-grip curls, Zottman curls, and bottoms-up hammer curls.

When it comes to visual appearance, the brachioradialis is one of the most noticeable areas of the forearms, particularly in a relaxed position.

Related:  More Bottom-Up Exercises

Related:  The Very Best Way to Build Forearms